Preliminary tests were carried out using ammonium thiosulphate as a chemical thinning agent for apple ('Cox's Orange Pippin' and 'Braeburn') blossoms. Ethephon and NAA (1-napthylacetic acid) were included for comparison. Whole tree sprays of 37g/l ammonium thiosulphate over-thinned 'Cox's Orange Pippin' blossoms and severely scorched blossoms, foliage, and apical meristems. Ethephon at 0.35 g/l also over-thinned, and NAA thinned to an intermediate extent when compared with the controls. When the lower concentration of 3.7 g/l ammonium thiosulphate was directly applied to stamens and styles of 'Braeburn' blossoms by brush, initial fruit set was only 30% that of untreated blossoms. When 0.35 g/I ethephon was directly applied by brush to spur leaves or petals of 'Braeburn' blossoms at pink bud, initial fruit set was only 23% that of untreated blossoms. lt is concluded that ammonium thiosulphate has the potential to thin apple blossoms. Further experiments to define optimum concentrations and spray volumes are needed

Drost, J. H.

Irving, D. E.

Pallesen, J. C.

D. E. Irving

J. C. Pallesen

J. H. Drost

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New Zealand Journal of Crop and Horticultural Science

Preliminary results on chemical thinning of apple blossoms with ammonium thiosulphate, NAA, and ethephon

University of Queensland eSpace

Short Communication:  
Preliminary results on chemical thinning of apple blossoms with 
ammonium thiosulphate, NAA and ethephon  
 
D.E. Irving, J.C. Pallesen, J.H. Drost  
MAFTechnology, Levin Horticultural Research Centre, Private Bag, Levin, New 
Zealand  
 
Abstract  
 
Preliminary tests were carried out using ammonium thiosulphate as a chemical 
thinning agent for apple ('Cox's Orange Pippin' and 'Braeburn') blossoms. 
Ethephon and NAA (1-napthylacetic acid) were included for comparison. Whole 
tree sprays of 37g/l ammonium thiosulphate over-thinned 'Cox's Orange Pippin' 
blossoms and severely scorched blossoms, foliage, and apical meristems. 
Ethephon at 0.35 g/l also over-thinned, and NAA thinned to an intermediate 
extent when compared with the controls. When the lower concentration of 3.7 g/l 
ammonium thiosulphate was directly applied to stamens and styles of 'Braeburn' 
blossoms by brush, initial fruit set was only 30% that of untreated blossoms. 
When 0.35 g/I ethephon was directly applied by brush to spur leaves or petals of 
'Braeburn' blossoms at pink bud, initial fruit set was only 23% that of untreated 
blossoms. lt is concluded that ammonium thiosulphate has the potential to thin 
apple blossoms. Further experiments to define optimum concentrations and 
spray volumes are needed.  
 
Keywords apple; ammonium thiosulphate; ammonium thiosulfate; blossom 
sprays; chemical thinning; ethephon fruit set; NAA (1-napthylaceticacid); urea; 
spray damage  
 
INTRODUCTION  
 
The importance of chemical thinning in New Zealand apple orchards is wall 
recognized (Davison 1966). Chemical thinners remove large numbers of flowers 
fruitlets, and this reduces the hand thinning requirement promotes production of 
large fruit through early thinning, regularizes cropping and reduces branch 
breakage.  
 
The apple industry is the second largest horticultural industry in New Zealand 
and relies on NAA (1-napthylacetic acid) and carbaryl (1-napthyl-n-
methylcarbamate) for chemical thinning. It is now clear that overseas consumers 
demand reduced use of chemicals in food production, and NAA and carbaryl are 
no longer registered in West Germany. Although West Germany is not our sole 
market for apples, it seems pertinent to assess the value of other chemicals, and 
to seek, for the longer term, chemical thinning agents which leave no residues 
and are environmentally safe.  
To address these issues, we tested the effectiveness of ammonium thiosulphate 
as a chemical thinner, and included NAA and ethephon for comparison. 
Ammonium thiosulphate thins peach blooms (Byers & Lyons 1983; 1984), and is 
used in both pharmaceutical and food industries. Since there are no published 
results on the use of ammonium thiosulphate as a thinner for apple blossoms, we 
report our initial results on thinning action in this communication.  
 
MATERIALS AND METHODS  
 
Experimental sites and tree management  
The experiments were conducted on a Ministry of Agriculture and Fisheries 
experimental sand country orchard in the Manawatu District ('Cox's Orange 
Pippin') and at the Levin Horticultural Research Centre ('Braeburn'). Trees used 
were 6-year-old 'Cox's Orange Pippin' (Greenmeadows selection) on MM 106 
rootstock at a spacing of 5 x 3 m, and 2-year-old 'Braeburn' on MM 106 at a 
spacing of 5 x 1.8 m. All trees were trained to a centre leader.  
 
Experiment 1  
'Cox's Orange Pippin' trees with uniform butt circumference 30 cm above ground 
were selected. Sixty flower clusters (thinned to five flowers per cluster) on 5 
randomly selected replicate trees were tagged before spraying. On each tree, 20 
clusters were marked at each of 3 blossoming stages: (1) pink bud; (2) full bloom 
(when anthers were pale yellow); (3) full bloom/petal fall (when anthers were 
brown). Treatments applied are given in Table 1. Chemicals used were: 
ethephon (( 2-chloroethyl) phosphonic acid, Ivon Watkins-Dow Ltd); NAA (New 
Zealand Fruitgrowers Chemical Company Ltd); purified ammonium thiosulphate 
(Prolabo, Paris). All sprays (including the controls) included 0.1% (v/v) Regulaid® 
(Kalo Inc. Overland Park, KS 66211, USA), a non-ionic spreader which was 
applied with hand-held sprayers. Initial fruit set on labeled clusters was 
determined 4 weeks after the treatments were applied. Data were analyzed by 
analysis of variance.  
 
Experiment 2  
Ammonium thiosulphate (3.7 g/I) and ethephon (0.35 g/1), both in 0.1% 
Regulaid®, were applied to 'Braeburn' blossoms as follows: (1) petals, at pink 
bud; (2) petals, at full bloom; (3) stamens and styles, at full bloom; (4) spur 
leaves of a blossom cluster, at full bloom. Applications were made using a small 
brush to completely wet sites of application. Clusters were labeled and thinned to 
3 blossoms each before treatment. There were 5 single tree replicates of uniform 
size according to butt circumference 30 cm above ground for each chemical. 
There were 5 tissue treatments (including the controls which were sprayed with 
0.1% non-ionic spreader), with 4 replicate clusters per treatment per tree. Initial 
fruit set on labeled clusters was recorded 3 weeks after treatments were applied. 
Data were analysed by analysis of variance.  
 
 
RESULTS AND DISCUSSION  
 
In Experiment 1, chemicals which thinned blossoms did so at the pink bud, full 
bloom, and full bloom/petal fall stages equally (data not shown) and therefore 
pooled data for each treatment are given in Table 1. Ethephon, NAA, ethephon 
plus NAA, and ammonium thiosulphate (37 g/1) produced the greatest thinning 
effect on ‘Cox's Orange Pippin'. However, the chemicals over-thinned as the 
target was about 4.5 (transformed data), or 20 fruit set per 100 blossoms.  
Ammonium thiosulphate (37 g/I) caused petals to lose turgor within 2 hours of 
application. After 2 days, there was marked scorching of petals, apical 
meristems, and leaves, and these symptoms were still apparent after 16 days.  
 
Ammonium thiosulphate at 3.7 and 0.37 g/l had no thinning effect in this 
experiment, although 3.7 g/l ammonium thiosulphate caused mild burning of 
petals and foliage. Ammonium thiosulphate at 0.37 g/l produced no visible effects 
on trees. When applied directly to stamens and styles of 'Braeburn' blossoms, 
3.7 g/l ammonium thiosulphate effectively reduced fruit set (Table 2). Clearly, 
further research should examine thinning and side effects of concentrations 
between 3.7g/I and 37 g/I.  
 
Byers & Lyons (1985) suggested that desiccants interfere with fertilisation and/or 
produce phytotoxic effects in the peduncle leading to fruit abscission in peach. 
These mechanisms could explain the thinning action of ammonium thiosulphate 
in apple. 0ur observations draw us to conclude that 37 g/l ammonium 
thiosulphate thinned ‘Çox’s Orange Pippin' by completely desiccating the blooms, 
whilst in the `Braeburn' experiment, desiccation of styles may have been the 
mechanism of thinning.  
 
Ethephon had, as expected, a very good thinning effect (Table 1). Ethephon has 
been widely tested as a chemical thinning agent, but with different degrees of 
success (Jones et al. 1983; Knight 1980), possibly because there are two periods 
of sensitivity: blossoming, and near the main fruit drop period (June or December 
drop). In our experiment, ethephon was effective from the pink bud stage through 
to petal fall and was particularly effective when applied to petals at the pink bud 
stage and to leaves of 'Braeburn' (Table 2). NAA did not enhance the thinning 
effect of ethephon. Mavrodiev & Manolov (1987) and Schneider (1978) suggest 
that ethylene is not the primary cause of fruitlet abscission but that reduction in 
assimilate supply to fruitlets is the main reason for fruit drop. However, when 
applied to petals, ethephon presumably induces abscission of blossom parts. It is 
unlikely that the mechanism of ethephon thinning is the same for both foliage and 
petal application.  
 
Our preliminary data show ammonium thiosulphate has the potential to thin apple 
blossoms. In view of public attitudes towards insecticides (such as carbaryl) and 
growth regulators (such as NAA and ethephon), it is of interest to develop 
alternatives to such chemicals. For ammonium thiosulphate, it is necessary to 
define optimum application rates and spay volumes to reduce foliage injury and 
to improve thinning effectiveness. In addition, other desiccating compounds, such 
as urea, should be tested for thinning action.  
 
REFERENCES  
 
Byers, R.E.; Lyons, C.G. 1983: Chemical peach thinning with surfactants and 
ammonium nitrate Journal of Horticultural Science 58:517-519.  
Byers, R.E.; Lyons, C.G. 1984: Flower thinning of peach with desiccating 
chemicals. Hortscience 19: 545-546.  
Byers, R.E.; Lyons, C.G. 1985: Peach flower thinning and possible sites of action 
of desiccating chemicals. Journal of the American Society for Horticultural 
Science 110: 662-667.  
Davison. R.M. 1966: Chemical thinning. New Zealand Department of Scientific 
and Industrial Research Information Series No.60.  
Jones K. M.; K.oen, T. B.; Meredith R.J. 1987: Thinning Golden Delicious apples 
using ethephon sprays. Journal of Horticultural Science 58: 381-388.  
Knight. J. N. 1980: Fruit thinning of the apple cultivar 'Cox's Orange Pippin’. 
Journal of Horticultural Science 55:267-273.  
Mavrodiev, S.; Manolov, P.1987: Effects of Flordimex on photosynthetic intensity 
and 14C-assimilate distribution during fruitlet thinning of apples. Crop 
Physiology Abstracts 13: [3758].  
Schneider, G. W. 1978: Abscission mechanism studies with apple fruitlets. 
Journal of the American Society for Horticultural Science 103: 455-458.  
 
Table 1 Chemicals used, rate of application and initial fruit set per 100 blossoms 
(√(x+0.5) transformed figures), following application to 'Cox's Orange Pippin 
apple. ATS= ammonium thiosulphate.  
 
Chemical  Rate  (g/l)  
Fruit/100 blossoms  
(√ (x+0.5))  
Ethephon  0.35  1.7 
NAA  0.02  3.7 
Ethephon+NAA  0.35+0.02  2.6  
ATS  37  2.8 
ATS  3.7  6.6 
ATS  0.37  5.5 
Control   6.3 
SED   0.6  
 
 
 
Table 2 Initial fruit set on 'Braeburn' following application of 3.7 g/l ammonium 
thiosulphate (ATS) or 0.35 g/1 ethephon to blossom parts or leaves. ND = not 
determined.  
 
 Fruit/100 blossoms 
Tissue treated  ATS  Ethephon  
Petals    
pink bud  30.6  6.2  
full bloom  ND  16.6  
Stamens & styles  11 .9  ND  
Leaves  32.3  6.3 
Control  37.7  27.0 
SED  3.5  0.8 
 
 
 


Preliminary Results on Chemical Thinning of Apple Blossoms with Ammonium Thiosulphate, NAA, and Ethephon

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Preliminary Results on Chemical Thinning of Apple Blossoms with Ammonium Thiosulphate, NAA, and Ethephon

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